Water pollution occurs when harmful substances affect water quality in rivers, lakes, oceans, and groundwater. It has significant negative impacts on human health and ecosystems. The main causes are untreated sewage, industrial and agricultural waste, urban and stormwater runoff containing pathogens, chemicals, excess nutrients and other contaminants. Effective control requires treatment of domestic sewage and industrial wastewater before discharge, as well as management approaches to reduce non-point pollution from sources such as agriculture and construction sites.

1. Water pollution
Raw sewage and industrial waste flows across international borders New River passes
from Mexicali to Calexico, California.
Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans,
groundwater).
Water pollution affects plants and organisms living in these bodies of water; and, in
almost all cases the effect is damaging either to individual species and populations, but
also to the natural biological communities.
Water pollution occurs when pollutants are discharged directly or indirectly into water
bodies without adequate treatment to remove harmful compounds.
Contents
[hide]
• 1 Introduction
• 2 Water pollution categories
o 2.1 Point source pollution
o 2.2 Non-point source pollution
• 3 Groundwater pollution
• 4 Causes of water pollution
o 4.1 Pathogens
o 4.2 Chemical and other contaminants
o 4.3 Thermal pollution
• 5 Transport and chemical reactions of water pollutants
• 6 Measurement of water pollution
o 6.1 Sampling
o 6.2 Physical testing

2. o 6.3 Chemical testing
o 6.4 Biological testing
• 7 Control of water pollution
o 7.1 Domestic sewage
o 7.2 Industrial wastewater
o 7.3 Agricultural wastewater
o 7.4 Construction site stormwater
o 7.5 Urban runoff (stormwater)
• 8 See also
• 9 References
• 10 External links
[edit] Introduction
Millions depend on the polluted Ganges river.
Water pollution is a major problem in the global context. It has been suggested that it is
the leading worldwide cause of deaths and diseases,[1][2]
and that it accounts for the deaths
of more than 14,000 people daily.[2]
An estimated 700 million Indians have no access to a
proper toilet, and 1,000 Indian children die of diarrheal sickness every day.[3]
Some 90%
of China's cities suffer from some degree of water pollution,[4]
and nearly 500 million
people lack access to safe drinking water.[5]
In addition to the acute problems of water
pollution in developing countries, industrialized countries continue to struggle with
pollution problems as well. In the most recent national report on water quality in the
United States, 45 percent of assessed stream miles, 47 percent of assessed lake acres, and
32 percent of assessed bay and estuarine square miles were classified as polluted.[6]
Water is typically referred to as polluted when it is impaired by anthropogenic
contaminants and either does not support a human use, like serving as drinking water,
and/or undergoes a marked shift in its ability to support its constituent biotic
communities, such as fish. Natural phenomena such as volcanoes, algae blooms, storms,
and earthquakes also cause major changes in water quality and the ecological status of
water.

3. [edit] Water pollution categories
Surface water and groundwater have often been studied and managed as separate
resources, although they are interrelated.[7]
Sources of surface water pollution are
generally grouped into two categories based on their origin.
[edit] Point source pollution
Point source pollution - Shipyard - Rio de Janeiro.
Point source pollution refers to contaminants that enter a waterway through a discrete
conveyance, such as a pipe or ditch. Examples of sources in this category include
discharges from a sewage treatment plant, a factory, or a city storm drain. The U.S. Clean
Water Act (CWA) defines point source for regulatory enforcement purposes.[8]
The CWA
definition of point source was amended in 1987 to include municipal storm sewer
systems, as well as industrial stormwater, such as from construction sites.[9]
[edit] Non-point source pollution
Non-point source (NPS) pollution refers to diffuse contamination that does not originate
from a single discrete source. NPS pollution is often accumulative effect of small
amounts of contaminants gathered from a large area. The leaching out of nitrogen
compounds from agricultural land which has been fertilized is a typical example. Nutrient
runoff in stormwater from "sheet flow" over an agricultural field or a forest are also cited
as examples of NPS pollution.
Contaminated storm water washed off of parking lots, roads and highways, called urban
runoff, is sometimes included under the category of NPS pollution. However, this runoff
is typically channeled into storm drain systems and discharged through pipes to local
surface waters, and is a point source. However where such water is not channeled and
drains directly to ground it is a non-point source.
[edit] Groundwater pollution

4. Interactions between groundwater and surface water are complex. Consequently,
groundwater pollution, sometimes referred to as groundwater contamination, is not as
easily classified as surface water pollution.[7]
By its very nature, groundwater aquifers are
susceptible to contamination from sources that may not directly affect surface water
bodies, and the distinction of point vs. non-point source may be irrelevant. A spill of a
chemical contaminant on soil, located away from a surface water body, may not
necessarily create point source or non-point source pollution, but nonetheless may
contaminate the aquifer below. Analysis of groundwater contamination may focus on soil
characteristics and hydrology, as well as the nature of the contaminant itself. See
Hydrogeology.
[edit] Causes of water pollution
The specific contaminants leading to pollution in water include a wide spectrum of
chemicals, pathogens, and physical or sensory changes such as elevated temperature and
discoloration. While many of the chemicals and substances that are regulated may be
naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the
key in determining what is a natural component of water, and what is a contaminant.
Oxygen-depleting substances may be natural materials, such as plant matter (e.g. leaves
and grass) as well as man-made chemicals. Other natural and anthropogenic substances
may cause turbidity (cloudiness) which blocks light and disrupts plant growth, and clogs
the gills of some fish species.[10]
Many of the chemical substances are toxic. Pathogens can produce waterborne diseases
in either human or animal hosts. Alteration of water's physical chemistry includes acidity
(change in pH), electrical conductivity, temperature, and eutrophication. Eutrophication
is an increase in the concentration of chemical nutrients in an ecosystem to an extent that
increases in the primary productivity of the ecosystem. Depending on the degree of
eutrophication, subsequent negative environmental effects such as anoxia (oxygen
depletion) and severe reductions in water quality may occur, affecting fish and other
animal populations.
[edit] Pathogens
A manhole cover unable to contain a sanitary sewer overflow.

5. Coliform bacteria are a commonly-used bacterial indicator of water pollution, although
not an actual cause of disease. Other microorganisms sometimes found in surface waters
which have caused human health problems include:
• Burkholderia pseudomallei
• Cryptosporidium parvum
• Giardia lamblia
• Salmonella
• Novovirus and other viruses
• Parasitic worms (helminths).[11][12]
High levels of pathogens may result from inadequately treated sewage discharges.[13]
This
can be caused by a sewage plant designed with less than secondary treatment (more
typical in less-developed countries). In developed countries, older cities with aging
infrastructure may have leaky sewage collection systems (pipes, pumps, valves), which
can cause sanitary sewer overflows. Some cities also have combined sewers, which may
discharge untreated sewage during rain storms.[14]
Pathogen discharges may also be caused by poorly-managed livestock operations.
[edit] Chemical and other contaminants
Muddy river polluted by sediment. Photo courtesy of United States Geological Survey.
Contaminants may include organic and inorganic substances.
Organic water pollutants include:
• Detergents
• Disinfection by-products found in chemically disinfected drinking water, such as
chloroform
• Food processing waste, which can include oxygen-demanding substances, fats
and grease
• Insecticides and herbicides, a huge range of organohalides and other chemical
compounds

6. • Petroleum hydrocarbons, including fuels (gasoline, diesel fuel, jet fuels, and fuel
oil) and lubricants (motor oil), and fuel combustion byproducts, from stormwater
runoff[15]
• Tree and bush debris from logging operations
• Volatile organic compounds (VOCs), such as industrial solvents, from improper
storage. Chlorinated solvents, which are dense non-aqueous phase liquids
(DNAPLs), may fall to the bottom of reservoirs, since they don't mix well with
water and are denser.
• Various chemical compounds found in personal hygiene and cosmetic products
Inorganic water pollutants include:
• Acidity caused by industrial discharges (especially sulfur dioxide from power
plants)
• Ammonia from food processing waste
• Chemical waste as industrial by-products
• Fertilizers containing nutrients--nitrates and phosphates--which are found in
stormwater runoff from agriculture, as well as commercial and residential use[15]
• Heavy metals from motor vehicles (via urban stormwater runoff)[15][16]
and acid
mine drainage
• Silt (sediment) in runoff from construction sites, logging, slash and burn practices
or land clearing sites
Macroscopic pollution—large visible items polluting the water—may be termed
"floatables" in an urban stormwater context, or marine debris when found on the open
seas, and can include such items as:
• Trash (e.g. paper, plastic, or food waste) discarded by people on the ground, and
that are washed by rainfall into storm drains and eventually discharged into
surface waters
• Nurdles, small ubiquitous waterborne plastic pellets
• Shipwrecks, large derelict ships

7. Potrero Generating Station discharges heated water into San Francisco Bay.[17]
[edit] Thermal pollution
Main article: Thermal pollution
Thermal pollution is the rise or fall in the temperature of a natural body of water caused
by human influence. A common cause of thermal pollution is the use of water as a
coolant by power plants and industrial manufacturers. Elevated water temperatures
decreases oxygen levels (which can kill fish) and affects ecosystem composition, such as
invasion by new thermophilic species. Urban runoff may also elevate temperature in
surface waters.
Thermal pollution can also be caused by the release of very cold water from the base of
reservoirs into warmer rivers.
[edit] Transport and chemical reactions of water
pollutants
See also: Marine pollution
Most water pollutants are eventually carried by rivers into the oceans. In some areas of
the world the influence can be traced hundred miles from the mouth by studies using
hydrology transport models. Advanced computer models such as SWMM or the DSSAM
Model have been used in many locations worldwide to examine the fate of pollutants in
aquatic systems. Indicator filter feeding species such as copepods have also been used to
study pollutant fates in the New York Bight, for example. The highest toxin loads are not
directly at the mouth of the Hudson River, but 100 kilometers south, since several days
are required for incorporation into planktonic tissue. The Hudson discharge flows south

8. along the coast due to coriolis force. Further south then are areas of oxygen depletion,
caused by chemicals using up oxygen and by algae blooms, caused by
A polluted river draining an abandoned copper mine on Anglesey
excess nutrients from algal cell death and decomposition. Fish and shellfish kills have
been reported, because toxins climb the food chain after small fish consume copepods,
then large fish eat smaller fish, etc. Each successive step up the food chain causes a
stepwise concentration of pollutants such as heavy metals (e.g. mercury) and persistent
organic pollutants such as DDT. This is known as biomagnification, which is
occasionally used interchangeably with bioaccumulation.
Large gyres (vortexes) in the oceans trap floating plastic debris. The North Pacific Gyre
for example has collected the so-called "Great Pacific Garbage Patch" that is now
estimated at 100 times the size of Texas. Many of these long-lasting pieces wind up in the
stomachs of marine birds and animals. This results in obstruction of digestive pathways
which leads to reduced appetite or even starvation.
Many chemicals undergo reactive decay or chemically change especially over long
periods of time in groundwater reservoirs. A noteworthy class of such chemicals is the
chlorinated hydrocarbons such as trichloroethylene (used in industrial metal degreasing
and electronics manufacturing) and tetrachloroethylene used in the dry cleaning industry
(note latest advances in liquid carbon dioxide in dry cleaning that avoids all use of
chemicals). Both of these chemicals, which are carcinogens themselves, undergo partial
decomposition reactions, leading to new hazardous chemicals (including
dichloroethylene and vinyl chloride).
Groundwater pollution is much more difficult to abate than surface pollution because
groundwater can move great distances through unseen aquifers. Non-porous aquifers
such as clays partially purify water of bacteria by simple filtration (adsorption and
absorption), dilution, and, in some cases, chemical reactions and biological activity:
however, in some cases, the pollutants merely transform to soil contaminants.
Groundwater that moves through cracks and caverns is not filtered and can be transported
as easily as surface water. In fact, this can be aggravated by the human tendency to use
natural sinkholes as dumps in areas of Karst topography.

9. There are a variety of secondary effects stemming not from the original pollutant, but a
derivative condition. An example is silt-bearing surface runoff, which can inhibit the
penetration of sunlight through the water column, hampering photosynthesis in aquatic
plants.
[edit] Measurement of water pollution
Environmental Scientists preparing water autosamplers.
Water pollution may be analyzed through several broad categories of methods: physical,
chemical and biological. Most involve collection of samples, followed by specialized
analytical tests. Some methods may be conducted in situ, without sampling, such as
temperature. Government agencies and research organizations have published
standardized, validated analytical test methods to facilitate the comparability of results
from disparate testing events.[18]
[edit] Sampling
Sampling of water for physical or chemical testing can be done by several methods,
depending on the accuracy needed and the characteristics of the contaminant. Many
contamination events are sharply restricted in time, most commonly in association with
rain events. For this reason "grab" samples are often inadequate for fully quantifying
contaminant levels. Scientists gathering this type of data often employ auto-sampler
devices that pump increments of water at either time or discharge intervals.
Sampling for biological testing involves collection of plants and/or animals from the
surface water body. Depending on the type of assessment, the organisms may be
identified for biosurveys (population counts) and returned to the water body, or they may
be dissected for bioassays to determine toxicity.
[edit] Physical testing
Common physical tests of water include temperature, solids concentration like total
suspended solids (TSS) and turbidity.

10. [edit] Chemical testing
See also: water chemistry analysis and environmental chemistry
Water samples may be examined using the principles of analytical chemistry. Many
published test methods are available for both organic and inorganic compounds.
Frequently-used methods include pH, biochemical oxygen demand (BOD), chemical
oxygen demand (COD), nutrients (nitrate and phosphorus compounds), metals (including
copper, zinc, cadmium, lead and mercury), oil and grease, total petroleum hydrocarbons
(TPH), and pesticides.
[edit] Biological testing
Main article: Bioindicator
Biological testing involves the use of plant, animal, and/or microbial indicators to
monitor the health of an aquatic ecosystem.
For microbial testing of drinking water, see Bacteriological water analysis.
[edit] Control of water pollution
[edit] Domestic sewage
Main article: Sewage treatment
Deer Island Waste Water Treatment Plant serving Boston, Massachusetts and vicinity.
Domestic sewage is 99.9% pure water, the other 0.1% are pollutants. While found in low
concentrations, these pollutants pose risk on a large scale.[19]
In urban areas, domestic
sewage is typically treated by centralized sewage treatment plants. In the U.S., most of
these plants are operated by local government agencies, frequently referred to as publicly
owned treatment works (POTW). Municipal treatment plants are designed to control
conventional pollutants: BOD and suspended solids. Well-designed and operated systems
(i.e., secondary treatment or better) can remove 90 percent or more of these pollutants.
Some plants have additional sub-systems to treat nutrients and pathogens. Most

11. municipal plants are not designed to treat toxic pollutants found in industrial wastewater.
[20]
Cities with sanitary sewer overflows or combined sewer overflows employ one or more
engineering approaches to reduce discharges of untreated sewage, including:
• utilizing a green infrastructure approach to improve stormwater management
capacity throughout the system, and reduce the hydraulic overloading of the
treatment plant[21]
• repair and replacement of leaking and malfunctioning equipment[14]
• increasing overall hydraulic capacity of the sewage collection system (often a
very expensive option).
A household or business not served by a municipal treatment plant may have an
individual septic tank, which treats the wastewater on site and discharges into the soil.
Alternatively, domestic wastewater may be sent to a nearby privately-owned treatment
system (e.g. in a rural community).
[edit] Industrial wastewater
Main article: Industrial wastewater treatment
Dissolved air flotation system for treating industrial wastewater.
Some industrial facilities generate ordinary domestic sewage that can be treated by
municipal facilities. Industries that generate wastewater with high concentrations of
conventional pollutants (e.g. oil and grease), toxic pollutants (e.g. heavy metals, volatile
organic compounds) or other nonconventional pollutants such as ammonia, need
specialized treatment systems. Some of these facilities can install a pre-treatment system
to remove the toxic components, and then send the partially-treated wastewater to the
municipal system. Industries generating large volumes of wastewater typically operate
their own complete on-site treatment systems.
Some industries have been successful at redesigning their manufacturing processes to
reduce or eliminate pollutants, through a process called pollution prevention.
Heated water generated by power plants or manufacturing plants may be controlled with:

12. • cooling ponds, man-made bodies of water designed for cooling by evaporation,
convection, and radiation
• cooling towers, which transfer waste heat to the atmosphere through evaporation
and/or heat transfer
• cogeneration, a process where waste heat is recycled for domestic and/or
industrial heating purposes.[22]
[edit] Agricultural wastewater
Main article: Agricultural wastewater treatment
Riparian buffer lining a creek in Iowa
Nonpoint source controls
Sediment (loose soil) washed off fields is the largest source of agricultural pollution in
the United States.[10]
Farmers may utilize erosion controls to reduce runoff flows and
retain soil on their fields. Common techniques include contour plowing, crop mulching,
crop rotation, planting perennial crops and installing riparian buffers.[23][24]:pp. 4-95–4-96
Nutrients (nitrogen and phosphorus) are typically applied to farmland as commercial
fertilizer; animal manure; or spraying of municipal or industrial wastewater (effluent) or
sludge. Nutrients may also enter runoff from crop residues, irrigation water, wildlife, and
atmospheric deposition.[24]:p. 2-9
Farmers can develop and implement nutrient management
plans to reduce excess application of nutrients.[23][24]:pp. 4-37–4-38
To minimize pesticide impacts, farmers may use Integrated Pest Management (IPM)
techniques (which can include biological pest control) to maintain control over pests,
reduce reliance on chemical pesticides, and protect water quality.[25]

13. Confined Animal Feeding Operation in the United States
Point source wastewater treatment
Farms with large livestock and poultry operations, such as factory farms, are called
concentrated animal feeding operations or confined animal feeding operations in the
U.S. and are being subject to increasing government regulation.[26][27]
Animal slurries are
usually treated by containment in lagoons before disposal by spray or trickle application
to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal
wastes, as are anaerobic lagoons. Some animal slurries are treated by mixing with straw
and composted at high temperature to produce a bacteriologically sterile and friable
manure for soil improvement.
[edit] Construction site stormwater
Silt fence installed on a construction site.
Sediment from construction sites is managed by installation of:
• erosion controls, such as mulching and hydroseeding, and
• sediment controls, such as sediment basins and silt fences.[28]
Discharge of toxic chemicals such as motor fuels and concrete washout is prevented by
use of:
• spill prevention and control plans, and
• specially-designed containers (e.g. for concrete washout) and structures such as
overflow controls and diversion berms.[29]

14. [edit] Urban runoff (stormwater)
Main article: Urban runoff
Retention basin for controlling urban runoff
Effective control of urban runoff involves reducing the velocity and flow of stormwater,
as well as reducing pollutant discharges. Local governments use a variety of stormwater
management techniques to reduce the effects of urban runoff. These techniques, called
best management practices (BMPs) in the U.S., may focus on water quantity control,
while others focus on improving water quality, and some perform both functions.[30]
Pollution prevention practices include low impact development techniques, installation of
green roofs and improved chemical handling (e.g. management of motor fuels & oil,
fertilizers and pesticides).[31]
Runoff mitigation systems include infiltration basins,
bioretention systems, constructed wetlands, retention basins and similar devices.[32][33]
Thermal pollution from runoff can be controlled by stormwater management facilities
that absorb the runoff or direct it into groundwater, such as bioretention systems and
infiltration basins. Retention basins tend to be less effective at reducing temperature, as
the water may be heated by the sun before being discharged to a receiving stream.[30]:p. 5-58
See also: Green infrastructure